in that year, i thought the 318 and 340 cranks were the same ?
i have been wrong eleventybillion times in my life, so one more time being wrong wouldn't be a surprise...............
I have heard that the 340 cranks have bigger counterweights for the heavier bigger bore 340 pistons, not the same as 318
I have seen 318 cranks used in 340 motor with steel plates welded on the crank counterweights for increasing the weight for balancing
If they both had the same casting number, I might believe there the same but when they aren't there is a reason for that
BTW, I have used a 318-3 HD truck crank in a 340 race motor with really light SS pistons, that worked really well